The present embodiments relate to monitoring a high-frequency transmit device of a magnetic resonance tomography system with a transmit antenna system having a plurality of transmit channels.
The imaging of an examination object, referred to in the following as a patient, using a magnetic resonance tomography system operates essentially in three steps: First, in the region of a body region to be imaged, a powerful, stable, homogeneous magnetic field (e.g., a B0 field) and therefore a stable alignment of the magnetization of protons (e.g., proton spin) is generated in the body region in question. This stable alignment is changed by supplying electromagnetic high-frequency energy (e.g., emitting high-frequency pulses (the B1 field)). This energy stimulation is terminated again, and the nuclear resonance signals produced in the body are measured with the aid of suitable receive coils in order thus to obtain information about the tissue in the body region. A magnetic resonance tomography system, therefore, includes a plurality of interacting components. Each interacting component of the plurality of interacting components uses modern and complex technologies. A central element of a magnetic resonance tomography system is the high-frequency transmit device that is responsible, for example, for generating the high-frequency pulses to be radiated into a body region. The high-frequency pulses emitted by a high-frequency power amplifier of the high-frequency transmit device are conducted in this process to a transmit antenna system, with which the high-frequency pulses are radiated into a body region.
As magnetic resonance tomography systems have been developed and become established, limit values regulating the maximum high-frequency radiation into a human body have been set to provide patient safety. A typical limit value for this is the maximum permissible specific absorption rate (SAR) value. For example, the whole body SAR that the power absorbed by the patient in a time window averaged over 6 minutes is not to exceed a value of 4 W/kg. Measuring devices that may be used to measure the high-frequency power are provided in the magnetic resonance systems. Directional couplers may be used in the supply lines to the antenna system for this purpose. These decouple signals that may represent the power of the emitted high-frequency pulses in the form of voltage values. The measured values may be processed to obtain suitable control values therefrom. If these control values exceed limit values, the function of the high-frequency transmit device may be restricted. For example, the emitted power may be reduced, a measurement may be aborted completely, or other measures may be taken so that patient safety may be provided when there is any doubt.
Most magnetic resonance tomography systems currently available on the market contain just one transmit channel. The amplitude accuracy and temporal stability of the measuring devices used for the abovementioned monitoring of the outward HF pulses are already provided by an extensive design concept. In the meantime, however, an increasing number of magnetic resonance tomography systems with multichannel transmit architectures have come into use. In such transmit antenna systems with a plurality of transmit channels, high-frequency pulses may be emitted via the individual transmit channels independently of one another, overlying one another after emission and thus generating a precisely defined B1 field. The generation of the B1 field magnetization is, for example, based on the correct addition of the pulse sequences generated by the individual transmit channels. The B1 field distribution is therefore no longer just a function of the amplitudes of the pulses on the individual channels but also of their relative phases.
In DE 10 2008 063 630 B4, therefore, a method is described for monitoring a high-frequency transmit device of a magnetic resonance tomography system, in which the high-frequency voltage amplitudes on the individual transmit channels are regularly determined, and the control values are respectively formed, taking into account a scatter parameter matrix of the transmit antenna system. The function of the high-frequency transmit device is restricted if such a control value reaches or exceeds a predetermined limit control value. In principle, this method functions very well. It is to be provided that the individual components of the system (e.g., the measuring devices of the transmit channels in question) operate correctly. Not only are the amplitudes to be measured with absolute accuracy, but also, any phase displacements between the individual measuring devices assigned to the transmit channels are to be known correctly and do not change. Extensive calibration routines may be performed, or the measuring device or the components associated therewith may either be checked by a service engineer or may even be replaced and submitted to a factory calibration in a defined cycle.